The present invention relates to a lithium secondary battery, particularly to an anode material to be used for non-aqueous electrolyte secondary batteries and lithium secondary battery using said anode material.
In recent years, electronic audio and visual devices and personal computers have rapidly been made into portable and cordless forms and the need for small secondary batteries having light weight and high energy density as the power source has greatly increased.
In this sense, there is a great expectation for non-aqueous secondary batteries, particularly lithium secondary batteries, as a battery workable in having high voltage and high energy density. Intensive research and development works for obtaining a layer structure compound of LiCoO2, LiNiO2 or complex oxide wherein a part of the oxide being substituted with a transition metal element, capable of intercalating and deintercalating lithium as an active anode material which comply with the requirement as described above, have been carried out until today. Furthermore, as materials having no layer structure, researches on LiMn2O4 as a cheap material having high voltage as much as 4V similar to LiCoO2, etc. and on LiMnO2 having relatively-low voltage as much as 3V, both are hereinafter collectively called as lithium manganate, have been also carried out.
In synthesizing of lithium manganate, it is preferable to use a material with high purity as much as possible as a raw material, however, from cost point of view, it is preferable to use any of cheap natural manganese oxide, electrolyzed manganese oxide manganese carbonate, etc., which have been used for primary batteries in the past.
However, the lithium manganate obtained by simply adjusting the composition and then calcining it, has problems such as getting lower cycling property at both normal temperatures and high temperatures from 60 to 80xc2x0 C. and getting lower preserving properties.
It is an object of the present invention to improve charging and discharging cycle property of the anode material by intentionally causing the deficit of cations and anions by means of optimizing the condition for calcining, adding different types of elements or the like, in order to solve the problems as described above.
The inventors of the present invention found that such improvement of charging and discharging cycle property can be achieved by optimizing the condition for synthesizing lithium manganate, whereby increasing the deficit amount of cations and anions in the crystal structure thereof, and they have reached to accomplish the present invention.
The first invention of the present invention based on such finding is related to a Li-Mn type complex oxide compound having a spinel structure, characterized in that either or both values of the occupancy rate (gc) of cations in an unit lattice to be determined based on the composition, the lattice constant and the density and/or the occupancy rate of anions (ga) in an unit lattice to be determined based on the composition, the lattice constant and the density is 0.985 or less.
The second invention of the present invention is characterized in that, a part of cations in a spinel structure represented by a formula, LiMn2O4, of the Li-Mn type complex oxide compound having a spinel structure is substituted by at least one element selected from a group consisting of Na, K, Co, Al, Mg, Ti, Cr, Fe, Cu and Ni.
The third invention of the present invention is characterized in that, the Li Mn type complex oxide compound having a spinel structure is LiMn2 xAlxO4, which is substituted with Al.
The forth invention of the present invention as to a lithium secondary battery is characterized in that the spinel-type lithium manganate is used as the anode material to be used for a lithium secondary battery.
Now, the present invention is further described in detail with referring the best embodiments described below, however, it should be noted that the present invention shall not be construed within a scope of such embodiments.
The anode material for lithium secondary batteries according to the present invention is the material which can improve the cycle property of the battery when the occupation ratio of either or both of the cations or/and the anions described above is adjusted to a value 0.985 or less as shown in the examples described below.
In the present invention, as a representative example for the Li-Mn type complex oxide compound having a spinel structure, LiMn2O4 can be given, whereas as an example for a complex oxide compound wherein a part of cations therein being substituted, LiMn2 xAlxO4, which is substituted with Al, can be given, however, the anode material according to the present invention shall not be limited to those compounds.
As examples for the element other than Al to be used for substituting a part of the cations, any of elements such as Na, K, Co, Mg, Ti, Cr, Fe, Cu and Nl can be given.
The reason to add such elements given above is to strengthen the binding between a metal element in the spinel and oxygen, thereby preventing collapse of the crystal structure to be caused along with charging and discharging cycles.
In general, lithium manganate applicable for 4V-class lithium secondary batteries has spinel-type crystal structure.
The general formula for the spinel-type crystal structure is represented by a general formula of AB2O4, and lithium manganate is represented by a general formula of LiMn2O4, for example.
The lattice constant is defined here as the size of a minimum repeating unit (an unit lattice) of crystals.
In case of the cubic crystals in a spinel structure, only the lattice constant a is used as a parameter (b=c=a, xcex1=xcex2=xcex3=90xc2x0), and the lattice constant a of lithium manganate is approximately 8 xc3x85, for example.
In an ideal case, 8 times number of elements based on the number of elements existing in a general formula are contained in an unit lattice, namely 8 elements of Li, 16 elements of Mn and 32 elements of O are existing therein.
Therefore, in case that no existence of ion deficit is assumed, the density can be known by determining the lattice constant according to a diffraction method, while amount of the deficit can be determined based on the lattice constant and the density.
Hereunder, an example of the deficit amount caused by the difference in calcining temperature is shown. It should be noted that the example below is just an example and there is no constraint on the calcining temperatures to cause the deficit. In the present invention, the anode materials whose occupancy rate is less than a given range can be preferably used irrespective of pressure of the preparation conditions.
In case of LiMn2O4, when the composition of the starting material is Li1.00Mn2.00O4.00, the product prepared after calcining at 700xc2x0 C. will be Li0.97Mn1.00O3.93. In case of cations, the occupancy rate is obtained from an equation 0.97+1.96=2.93, which is then divided by 3 to give a ratio of 0.977. Whereas, in case of anions, the occupancy rate is obtained by dividing 3.03 by 4 to give a ratio of 0.985.
Further, when the product is prepared by calcining at 900xc2x0 C., the product becomes Li0.99Mn1.97O3.96. In case of cations, the occupancy rate is obtained from an equation, 0.99+1.97=2.96, which is then divided by 3 to give a ratio of 0.987. Whereas, in the case of anions, the occupancy rate is obtained by dividing 3.95 by 4 to give a ratio of 0.988.
As obvious from the data obtained at a high calcining temperature, the occupancy rate of ions exceeds a given value, which shows that the material has changed into non-preferable one.
In case of LiMn2-xAlzO4 of which part of cations having substituted with Al, Li0.95Mn1.04Al0.10O3.90 is obtained when the composition of the starting materials Li1.00Mn1.90Al0.10O4.00 and the starting material is prepared by actually calcining at 700xc2x0 C. The occupancy ratio of cations is calculated from an equation, 0.98+1.84+0.10=2.92, which is then divided by 3 to give a rate of 0.973, while the occupancy ratio of anions is obtained by dividing 3.95 by 4 to give a ratio of 0.975.
Whereas, when the LiMn2-xAl3O4 described above is prepared by calcining at 900xc2x0 C., Li0.99Mn1.86Al0.11O3.91 is obtained. The occupancy ratio for the cations is calculated from an equation, 0.99+1.86 +0.11=2.96, which is then divided by 3 to give a ratio of 0.987, while the occupancy ratio for the anions is calculated a 0.978 by dividing 3.91 by 4.
Namely, the occupancy rate of ions is determined as one more than a given value when the calcining temperature is high, which thus shows that material has changed into an undesired one.
Using the anode material for lithium secondary batteries according to the present invention, improvement in the cycle property of a battery can be expected as the anode material has an adequate amount of ion deficit as described above.
And, the invention of a lithium secondary battery according to the present invention is constituted by using the anode material for lithium secondary batteries described above as an anode active material.
Whereas, in the present invention, as a cathode material usable for the lithium secondary battery according to the present invention, any materials capable of absorbing, storing and releasing metal lithium and lithium can be used without limitation, while as an electrolyte, any solvents, such as an organic solvent, including carbonates, sulfolanes, lactones and ethers, which contains a lithium salt, and a lithium ion conductive solid electrolyte, can be used without limitation.